Perivascular spaces, plasma GFAP, and speeded executive function in neurodegenerative diseases.

INTRODUCTION: We investigated the effect of perivascular spaces (PVS) volume on speeded executive function (sEF), as mediated by white matter hyperintensities (WMH) volume and plasma glial fibrillary acidic protein (GFAP) in neurodegenerative diseases. METHODS: A mediation analysis was performed to assess the relationship between neuroimaging markers and plasma biomarkers on sEF in 333 participants clinically diagnosed with Alzheimer's disease/mild cognitive impairment, frontotemporal dementia, or cerebrovascular disease from the Ontario Neurodegenerative Disease Research Initiative. RESULTS: PVS was significantly associated with sEF (c = -0.125 ± 0.054, 95% bootstrap confidence interval [CI] [-0.2309, -0.0189], p = 0.021). This effect was mediated by both GFAP and WMH. DISCUSSION: In this unique clinical cohort of neurodegenerative diseases, we demonstrated that the effect of PVS on sEF was mediated by the presence of elevated plasma GFAP and white matter disease. These findings highlight the potential utility of imaging and plasma biomarkers in the current landscape of therapeutics targeting dementia. HIGHLIGHTS: Perivascular spaces (PVS) and white matter hyperintensities (WMH) are imaging markers of small vessel disease. Plasma glial fibrillary protein acidic protein (GFAP) is a biomarker of astroglial injury. PVS, WMH, and GFAP are relevant in executive dysfunction from neurodegeneration. PVS's effect on executive function was mediated by GFAP and white matter disease.

Engaging Mood Brain Circuits with Psilocybin (EMBRACE): a study protocol for a randomized, placebo-controlled and delayed-start, neuroimaging trial in depression.

BACKGROUND: Major depressive disorder (MDD) is a leading cause of disability worldwide across domains of health and cognition, affecting overall quality of life. Approximately one third of individuals with depression do not fully respond to treatments (e.g., conventional antidepressants, psychotherapy) and alternative strategies are needed. Recent early phase trials suggest psilocybin may be a safe and efficacious intervention with rapid-acting antidepressant properties. Psilocybin is thought to exert therapeutic benefits by altering brain network connectivity and inducing neuroplastic changes that endure for weeks post-treatment. Although early clinical results are encouraging, psilocybin's acute neurobiological effects on neuroplasticity have not been fully investigated. We aim to examine for the first time how psilocybin acutely (intraday) and subacutely (weeks) alters functional brain networks implicated in depression. METHODS: Fifty participants diagnosed with MDD or persistent depressive disorder (PDD) will be recruited from a tertiary mood disorders clinic and undergo 1:1 randomization into either an experimental or control arm. Participants will be given either 25 mg psilocybin or 25 mg microcrystalline cellulose (MCC) placebo for the first treatment. Three weeks later, those in the control arm will transition to receiving 25 mg psilocybin. We will investigate whether treatments are associated with changes in arterial spin labelling and blood oxygenation level-dependent contrast neuroimaging assessments at acute and subacute timepoints. Primary outcomes include testing whether psilocybin demonstrates acute changes in (1) cerebral blood flow and (2) functional brain activity in networks associated with mood regulation and depression when compared to placebo, along with changes in MADRS score over time compared to placebo. Secondary outcomes include changes across complementary clinical psychiatric, cognitive, and functional scales from baseline to final follow-up. Serum peripheral neurotrophic and inflammatory biomarkers will be collected at baseline and follow-up to examine relationships with clinical response, and neuroimaging measures. DISCUSSION: This study will investigate the acute and additive subacute neuroplastic effects of psilocybin on brain networks affected by depression using advanced serial neuroimaging methods. Results will improve our understanding of psilocybin's antidepressant mechanisms versus placebo response and whether biological measures of brain function can provide early predictors of treatment response. TRIAL REGISTRATION: ClinicalTrials.gov Identifier: NCT06072898. Registered on 6 October 2023.

Treatment expectations and clinical outcomes following repetitive transcranial magnetic stimulation for treatment-resistant depression.

BACKGROUND: Patient expectations, including both positive (placebo) and negative (nocebo) effects, influence treatment outcomes, yet their impact on acute repetitive transcranial magnetic stimulation (rTMS) for treatment-resistant depression (TRD) is unclear. METHODS: In this single-center retrospective chart review, 208 TRD patients completed the Stanford Expectation of Treatment Scale (SETS) before starting open-label rTMS treatment. Patients were offered two excitatory rTMS protocols (deep TMS or intermittent theta-burst stimulation), which stimulated the left dorsolateral prefrontal cortex. A minimum of 20 once daily treatments were provided, delivered over 4-6 weeks. Primary outcomes were 1) remission, measured by a post-treatment score of <8 on the Hamilton Depression Rating Scale (HAMD-17), and 2) premature discontinuation. The change in HAMD-17 scores over time was used as a secondary outcome. Physicians were blinded to SETS scores. Logistic and linear regression, adjusting for covariates, assessed SETS and HAMD-17 relationships. RESULTS: Of 208 patients, 177 had baseline and covariate data available. The mean positivity bias score (positive expectancy minus negative expectancy subscale averages) was 0.48 ± 2.21, indicating the cohort was neutral regarding the expectations of their treatment on average. Higher positive expectancy scores were significantly associated with greater odds of remission (OR = 1.90, p = 0.003) and greater reduction in HAMD-17 scores (ß = 1.30, p = 0.005) at the end of acute treatment, after adjusting for covariates. Negative expectancy was not associated with decreased odds of remission (p = 0.2) or treatment discontinuation (p = 0.8). CONCLUSIONS: Higher pre-treatment positive expectations were associated with greater remission rates with open-label rTMS in a naturalistic cohort of patients with TRD.

Insertional effect following electrode implantation: an underreported but important phenomenon.

Deep brain stimulation has revolutionized the treatment of movement disorders and is gaining momentum in the treatment of several other neuropsychiatric disorders. In almost all applications of this therapy, the insertion of electrodes into the target has been shown to induce some degree of clinical improvement prior to stimulation onset. Disregarding this phenomenon, commonly referred to as 'insertional effect', can lead to biased results in clinical trials, as patients receiving sham stimulation may still experience some degree of symptom amelioration. Similar to the clinical scenario, an improvement in behavioural performance following electrode implantation has also been reported in preclinical models. From a neurohistopathologic perspective, the insertion of electrodes into the brain causes an initial trauma and inflammatory response, the activation of astrocytes, a focal release of gliotransmitters, the hyperexcitability of neurons in the vicinity of the implants, as well as neuroplastic and circuitry changes at a distance from the target. Taken together, it would appear that electrode insertion is not an inert process, but rather triggers a cascade of biological processes, and, as such, should be considered alongside the active delivery of stimulation as an active part of the deep brain stimulation therapy.

Associations Between Age at Menopause, Vascular Risk, and 3-Year Cognitive Change in the Canadian Longitudinal Study on Aging.

BACKGROUND AND OBJECTIVES: Mounting evidence supports sex differences in Alzheimer disease (AD) risk. Vascular and hormonal factors may together contribute to AD risk in female adults. We investigated whether age at menopause, vascular risk, and history of hormone therapy (HT) containing estrogens together influence cognition over a 3-year follow-up period. We hypothesized that earlier menopause and elevated vascular risk would have a synergistic association with lower cognitive scores at follow-up and that HT containing estrogens would attenuate this synergistic association to preserve cognition. METHODS: We used data from postmenopausal female participants and age-matched male participants in the Canadian Longitudinal Study on Aging. Vascular risk was calculated using a summary score of elevated blood pressure, antihypertensive medications, elevated low-density lipoprotein cholesterol, diabetes, smoking, and obesity. Cognition was measured with a global cognitive composite at baseline and 3-year follow-up. Linear models tested independent and interactive associations of age at menopause, vascular risk, and HT history with cognition at 3-year follow-up, adjusting for baseline cognition, baseline age, years of education, and test language (English/French). RESULTS: We included 8,360 postmenopausal female participants (mean age at baseline = 65.0 ± 8.53 years, mean age at menopause = 50.1 ± 4.62 years) and 8,360 age-matched male participants for comparison. There was an interaction between age at menopause and vascular risk, such that earlier menopause and higher vascular risk were synergistically associated with lower cognitive scores at follow-up (ß = 0.013, 95% CI 0.001-0.025, p = 0.03). In stratified analyses, vascular risk was associated with lower cognitive scores in female participants with earlier menopause (menopausal ages 35-48 years; ß = -0.044, 95% CI -0.066 to -0.022, p < 0.001), but not average (ages 49-52 years; ß = -0.007, 95% CI -0.027 to 0.012, p = 0.46) or later menopause (ages 53-65 years; ß = 0.003, 95% CI -0.020 to 0.025, p = 0.82). The negative association of vascular risk with cognition in female participants with earlier menopause was stronger than the equivalent association in age-matched male participants. HT history did not further modify the synergistic association of age at menopause and vascular risk with follow-up cognition (ß = -0.005, 95% CI -0.032 to 0.021, p = 0.69). DISCUSSION: Endocrine and vascular processes may synergistically contribute to increased risk of cognitive decline in female adults. These findings have implications for the development of sex-specific dementia prevention strategies.

Investigating the impact of hypertension with and without diabetes on Alzheimer's disease risk: A clinico-pathological study.

INTRODUCTION: Hypertension and diabetes are common cardiovascular risk factors that increase Alzheimer's disease (AD) risk. However, it is unclear whether AD risk differs in hypertensive individuals with and without diabetes. METHODS: Cognitively normal individuals (N = 11,074) from the National Alzheimer's Coordinating Center (NACC) were categorized as having (1) hypertension with diabetes (HTN+/DM+), (2) hypertension without diabetes (HTN+/DM-), or (3) neither (HTN-/DM-). AD risk in HTN+/DM+ and HTN+/DM- was compared to HTN-/DM-. This risk was then investigated in those with AD neuropathology (ADNP), cerebral amyloid angiopathy (CAA), cerebrovascular neuropathology (CVNP), arteriolosclerosis, and atherosclerosis. Finally, AD risk in HTN-/DM+ was compared to HTN-/DM-. RESULTS: Seven percent (N = 830) of individuals developed AD. HTN+/DM+ (hazard ratio [HR] = 1.31 [1.19-1.44]) and HTN+/DM- (HR = 1.24 [1.17-1.32]) increased AD risk compared to HTN-/DM-. AD risk was greater in HTN+/DM+ with ADNP (HR = 2.10 [1.16-3.79]) and CAA (HR = 1.52 [1.09-2.12]), and in HTN+/DM- with CVNP (HR = 1.54 [1.17-2.03]). HTN-/DM+ also increased AD risk (HR = 1.88 [1.30-2.72]) compared to HTN-/DM-. DISCUSSION: HTN+/DM+ and HTN+/DM- increased AD risk compared to HTN-/DM-, but pathological differences between groups suggest targeted therapies may be warranted based on cardiovascular risk profiles. HIGHLIGHTS: AD risk was studied in hypertensive (HTN+) individuals with/without diabetes (DM+/-). HTN+/DM+ and HTN+/DM- both had an increased risk of AD compared to HTN-/DM-. Post mortem analysis identified neuropathological differences between HTN+/DM+ and HTN+/DM-. In HTN+/DM+, AD risk was greater in those with AD neuropathology and CAA. In HTN+/DM-, AD risk was greater in those with cerebrovascular neuropathology.

Adverse Effects of Deep Brain Stimulation for Treatment-Resistant Depression: A Scoping Review.

Deep brain stimulation (DBS) is an emerging therapy for treatment-resistant depression (TRD). Although adverse effects have been reported in early-phase and a few randomized clinical trials, little is known about its overall safety profile, which has been assumed to be similar to that of DBS for movement disorders. The objective of this study was to pool existing safety data on DBS for TRD. Following PRISMA guidelines, PubMed was searched for English articles describing adverse outcomes after DBS for TRD. Studies were included if they reported at least 5 patients with a minimal follow-up of 6 months. After abstract (n = 607) and full-article review (n = 127), 28 articles reporting on 353 patients met criteria for final inclusion. Follow-up of the studies retrieved ranged from 12 to 96 months. Hemorrhages occurred in 0.8% of patients and infections in 10.2%. The rate of completed suicide was 2.5%. Development or worsening of depressive symptoms, anxiety, and mania occurred in 18.4%, 9.1%, and 5.1%, respectively. There were some differences between targets, but between-study heterogeneity precluded statistical comparisons. In conclusion, DBS for TRD is associated with surgical and psychiatric adverse events. Hemorrhage and infection occur at rates within an accepted range for other DBS applications. The risk of suicide after DBS for TRD is 2.5% but may not represent a significant deviation from the natural history of TRD. Finally, risks of worsening depression, anxiety, and the incidence of mania should be acknowledged when considering DBS for TRD.

Plasma VEGFA and PGF impact longitudinal tau and cognition in preclinical Alzheimer's disease.

Vascular dysfunction is increasingly recognized as an important contributor to the pathogenesis of Alzheimer's disease. Alterations in vascular endothelial growth factor (VEGF) pathways have been implicated as potential mechanisms. However, the specific impact of VEGF proteins in preclinical Alzheimer's disease and their relationships with other Alzheimer's disease and vascular pathologies during this critical early period remain to be elucidated. We included 317 older adults from the Harvard Aging Brain Study, a cohort of individuals who were cognitively unimpaired at baseline and followed longitudinally for up to 12 years. Baseline VEGF family protein levels (VEGFA, VEGFC, VEGFD, PGF and FLT1) were measured in fasting plasma using high-sensitivity immunoassays. Using linear mixed effects models, we examined the interactive effects of baseline plasma VEGF proteins and amyloid PET burden (Pittsburgh Compound-B) on longitudinal cognition (Preclinical Alzheimer Cognitive Composite-5). We further investigated if effects on cognition were mediated by early neocortical tau accumulation (flortaucipir PET burden in the inferior temporal cortex) or hippocampal atrophy. Lastly, we examined the impact of adjusting for baseline cardiovascular risk score or white matter hyperintensity volume. Baseline plasma VEGFA and PGF each showed a significant interaction with amyloid burden on prospective cognitive decline. Specifically, low VEGFA and high PGF were associated with greater cognitive decline in individuals with elevated amyloid, i.e. those on the Alzheimer's disease continuum. Concordantly, low VEGFA and high PGF were associated with accelerated longitudinal tau accumulation in those with elevated amyloid. Moderated mediation analyses confirmed that accelerated tau accumulation fully mediated the effects of low VEGFA and partially mediated (31%) the effects of high PGF on faster amyloid-related cognitive decline. The effects of VEGFA and PGF on tau and cognition remained significant after adjusting for cardiovascular risk score or white matter hyperintensity volume. There were concordant but non-significant associations with longitudinal hippocampal atrophy. Together, our findings implicate low VEGFA and high PGF in accelerating early neocortical tau pathology and cognitive decline in preclinical Alzheimer's disease. Additionally, our results underscore the potential of these minimally-invasive plasma biomarkers to inform the risk of Alzheimer's disease progression in the preclinical population. Importantly, VEGFA and PGF appear to capture distinct effects from vascular risks and cerebrovascular injury. This highlights their potential as new therapeutic targets, in combination with anti-amyloid and traditional vascular risk reduction therapies, to slow the trajectory of preclinical Alzheimer's disease and delay or prevent the onset of cognitive decline.

Insulin-like growth factor-1 and cognition in normoglycemia, prediabetes, and type 2 diabetes mellitus.

BACKGROUND: The relationship between insulin-like growth factor-1 (IGF-1) and cognition has been studied in healthy individuals, but not extensively with regards to insulin resistance and type 2 diabetes mellitus (T2DM). In this retrospective observational study, we investigated relationships of IGF-1 with memory and executive function across people with normoglycemia, prediabetes, and T2DM. METHODS: Data from the Midlife in the United States (MIDUS) study were used. Episodic memory and executive function were assessed using the Brief Test of Adult Cognition by Telephone approximately 21.42 ± 12.10 months prior to measuring IGF-1 levels from a fasting blood sample. Normoglycemia was identified as individuals without a physician diagnosis of diabetes and glycated hemoglobin (HbA1c) ≤5.6%. Prediabetes was identified as those without a physician diagnosis of diabetes and HbA1c between 5.7%-6.4%. T2DM was identified as anyone with a physician diagnosis of diabetes, or HbA1c ≥6.5%, or anyone using an oral hypoglycemic medication. The associations were assessed using linear regressions controlling for age, sex, education, body mass index, C-reactive protein, HbA1c or homeostatic model of insulin resistance, MIDUS wave, exercise, smoking status, sleep quality, alcohol intake, oral hypoglycemic use, and insulin use. RESULTS: The study included 1400 participants, which consisted of 583 normoglycemic (48.4% female, mean age 51.0 ± 12.2 years), 512 prediabetes (58.4% female, mean age 57.3 ± 11.8 years), and 305 T2DM participants (53.8% female, mean age 57.6 ± 11.5 years). Peripheral IGF-1 concentrations were lower (F2,1397 = 28.29, p < 0.001) in people with prediabetes or T2DM, vs. normoglycemia. Participants with prediabetes or T2DM had lower episodic memory (F2,1397 = 9.21, p < 0.001) and executive function (F2,1397 = 20.29, p < 0.001) composite z-scores than people with normoglycemia. Higher IGF-1 concentrations were associated with better executive performance in individuals with prediabetes (ß = 0.115 [0.028, 0.202], p = 0.010), but not in individuals with normoglycemia or T2DM. An interaction between IGF-1 and sex in predicting executive function was observed in the prediabetes group (ß = -0.344, p = 0.042), where the relationship was weaker in females (ß = 0.106 [-0.012, 0.224], p = 0.077) than males (ß = 0.251 [0.123, 0.380], p < 0.001). No associations were seen between IGF-1 and memory. CONCLUSION: The results suggest that peripheral IGF-1 concentrations may be related to executive function, and that the relationship may be sex-specific and dependent on diabetes status.

Ventromedial Prefrontal Cortex Does Not Play a Selective Role in Pattern Separation.

Humans have the capacity to form new memories of events that are, at times, highly similar to events experienced in the past, as well as the capacity to integrate and associate new information within existing knowledge structures. The former process relies on mnemonic discrimination and is believed to depend on hippocampal pattern separation, whereas the latter is believed to depend on generalization signals and conceptual categorization supported by the neocortex. Here, we examine whether and how the ventromedial prefrontal cortex (vMPFC) supports discrimination and generalization on a widely used task that was primarily designed to tax hippocampal processes. Ten individuals with lesions to the vMPFC and 46 neurotypical control participants were administered an adapted version of the mnemonic similarity task [Stark, S. M., Yassa, M. A., Lacy, J. W., & Stark, C. E. L. A task to assess behavioral pattern separation (BPS) in humans: Data from healthy aging and mild cognitive impairment. Neuropsychologia, 51, 2442-2449, 2013], which assesses the ability to distinguish previously learned images of everyday objects (targets) from unstudied, highly similar images (lures) and dissimilar images (foils). Relative to controls, vMPFC-lesioned individuals showed intact discrimination of lures from targets but a propensity to mistake studied targets and similar lures for dissimilar foils. This pattern was accompanied by inflated confidence despite low accuracy when responding to similar lures. These findings demonstrate a more general role of the vMPFC in memory retrieval, rather than a specific role in supporting pattern separation.

A Blood-Based Lipid Profile Associated With Hippocampal Volume and Brain Resting-State Activation Within Obese Adults from the UK Biobank.

Objectives: Obesity and dyslipidemia may be associated with hippocampal alterations and may increase the risk of neurodegeneration. This study studied hippocampal anatomical and functional association with a lipid profile based on high-density lipoprotein, low-density lipoprotein, and triglyceride related to dyslipidemia in obese and nonobese adults. A whole-brain analysis was also conducted to examine the effect of dyslipidemia on resting-state function across the brain. Participants and Methods: In total, 553 UK Biobank participants comprised three groups based on body mass index (BMI) rankings: obese adults with high BMI (OHigh, n = 184, 32.7 kg/m2 ≤ BMI ≤53.4 kg/m2), obese adults with a lower BMI (OLow, n = 182, 30.3 kg/m2 ≤ BMI ≤32.6 kg/m2), and nonobese controls (n = 187). Structural MRI and functional MRI data were accessed. The fractional amplitude of low-frequency fluctuations (fALFFs) maps was calculated to reflect resting-state brain activity. A lipid health factor was created using principal component analysis. Linear models tested for associations between the lipid health score and hippocampal MRI readouts. Results: With a higher lipid health factor corresponding to a lower dyslipidemia risk, we found a positive correlation between hippocampal volume with the lipid health factor exclusively in group OLow (p = 0.01). We also found a positive association between the lipid health factor and hippocampal fALFF in group OHigh (p = 0.02). Additional fALFF voxel-wise analysis to group OHigh also implicated that the premotor cortex, amygdala, thalamus, subcallosal cortex, temporal fusiform cortex, and middle temporal gyrus brain regions are related with lipid. Conclusion: The study finds novel associations among circulating lipid, hippocampal structure, and hippocampal function exclusively in the obese adults.

Persistent post-COVID headache is associated with suppression of scale-free functional brain dynamics in non-hospitalized individuals.

INTRODUCTION: Post-acute coronavirus disease 2019 (COVID-19) syndrome (PACS) is a growing concern, with headache being a particularly debilitating symptom with high prevalence. The long-term effects of COVID-19 and post-COVID headache on brain function remain poorly understood, particularly among non-hospitalized individuals. This study focused on the power-law scaling behavior of functional brain dynamics, indexed by the Hurst exponent (H). This measure is suppressed during physiological and psychological distress and was thus hypothesized to be reduced in individuals with post-COVID syndrome, with greatest reductions among those with persistent headache. METHODS: Resting-state blood oxygenation level-dependent (BOLD) functional magnetic resonance imaging data were collected for 57 individuals who had COVID-19 (32 with no headache, 14 with ongoing headache, 11 recovered) and 17 controls who had cold and flu-like symptoms but  tested negative for COVID-19. Individuals were assessed an average of 4-5 months after COVID testing, in a cross-sectional, observational study design. RESULTS: No significant differences in H values were found between non-headache COVID-19 and control groups., while those with ongoing headache had significantly reduced H values, and those who had recovered from headache had elevated H values, relative to non-headache groups. Effects were greatest in temporal, sensorimotor, and insular brain regions. Reduced H in these regions was also associated with decreased BOLD activity and local functional connectivity. CONCLUSIONS: These findings provide new insights into the neurophysiological mechanisms that underlie persistent post-COVID headache, with reduced BOLD scaling as a potential biomarker that is specific to this debilitating condition.

Seasonal pattern and depression outcomes from repetitive transcranial magnetic stimulation.

Individuals with major depressive disorder (MDD) may exhibit a seasonal pattern. The impact of a seasonal pattern in depressive symptoms on rTMS outcomes is unexplored. A retrospective analysis was performed on patients with MDD receiving open-label high frequency rTMS to the left dorsolateral prefrontal cortex. Having a seasonal pattern was defined as scoring ≥ 12 on the Personal Inventory for Depression and Seasonal Affective Disorder (PIDS). Primary outcomes included improvement in the Hamilton Depression Rating Scale (HAMD) and remission. Secondary analyses included the use of the self-rated Quick Inventory of Depressive Symptomatology (QIDS) to assess for changes in atypical neurovegetative symptoms. Multiple linear regression, multiple logistic regression, and linear mixed effects analyses were performed. 46 % (58/127) of the sample had a seasonal pattern. Seasonal pattern did not significantly influence improvement in HAMD (PIDS < 12, 7.8, SD 5.9; PIDS ≥ 12, 10.4, SD 4.9 or remission (PIDS < 12, 30 %; PIDS ≥ 12, 34 %). There were equivalent degrees of improvement in atypical neurovegetative symptoms over time as assessed using the QIDS. Depression with seasonal pattern was found to respond to rTMS treatment similarly to depression without seasonal pattern, suggesting that this may be a viable treatment for this group.

How Do Modifiable Risk Factors Affect Alzheimer's Disease Pathology or Mitigate Its Effect on Clinical Symptom Expression?

Epidemiological studies show that modifiable risk factors account for approximately 40% of the population variability in risk of developing dementia, including sporadic Alzheimer's disease (AD). Recent findings suggest that these factors may also modify disease trajectories of people with autosomal-dominant AD. With positron emission tomography imaging, it is now possible to study the disease many years before its clinical onset. Such studies can provide key knowledge regarding pathways for either the prevention of pathology or the postponement of its clinical expression. The former "resistance pathway" suggests that modifiable risk factors could affect amyloid and tau burden decades before the appearance of cognitive impairment. Alternatively, the resilience pathway suggests that modifiable risk factors may mitigate the symptomatic expression of AD pathology on cognition. These pathways are not mutually exclusive and may appear at different disease stages. Here, in a narrative review, we present neuroimaging evidence that supports both pathways in sporadic AD and autosomal-dominant AD. We then propose mechanisms for their protective effect. Among possible mechanisms, we examine neural and vascular mechanisms for the resistance pathway. We also describe brain maintenance and functional compensation as bases for the resilience pathway. Improved mechanistic understanding of both pathways may suggest new interventions.